1. Field of the Invention
This invention relates to a video signal processing apparatus. More specifically, this invention relates to a video signal processing apparatus for processing a video signal to reduce flicker.
2. Description of the Prior Art
A video signal processing apparatus for processing a video signal in which flicker due to illumination is suppressed with an automatic gain controlling circuit is known.
When image is taken by a video camera under illumination by fluorescent lamps, there is a problem of flicker. A luminance level of the fluorescent lamp periodically changes with a voltage of the ac line. FIGS. 3A and 3B are illustrations of prior art showing voltage change and luminance change of fluorescent lamps. If the cycle of the voltage of ac line is 50 Hz as shown in FIG. 3A, the luminance of a fluorescent lamp changes at 100 Hz as shown in FIG. 3B. If the cycle of the voltage of an ac line is 60 Hz, the luminance of the fluorescent lamp changes at 120 Hz as well known.
FIGS. 3C and 3D are illustrations showing operation of a prior art video camera. If a video camera or an electronic camera employing a MOS type of imager is used under illumination by fluorescent lamps at a shutter speed of {fraction (1/30)} sec, charge storing timings and the luminance level of the video signal are shown in FIGS. 3C and 3D.
The MOS type of imager outputs a first line from timing A1 to B1 and a second line from timings A2 to B2 which timings A2 and B2 are slightly shifted in time base from the timings A1 and B1. The last line (525th line) is detected at timings A525 and B525 within the one frame ({fraction (1/30)} sec). As shown in FIG. 3B, the luminance level changes, so that a luminance level of the image changes at a cycle of {fraction (1/100)} sec, which is sensed by a watcher as flicker. Particularly, in the case of the MOS type of imager, the reproduced image shows stripes over the to-be-reproduced image because the image storing timings are different every line. In the case of the CCD type of imager, because image storing timings are the same over all lines in the frame, flicker does not occur within the frame. However, the luminance signal level changes every frames in the CCD type of imager.
FIG. 4 is a block diagram of a prior art video signal processing apparatus which can suppress flicker. An output signal of an imager 11 is supplied to an automatic gain control circuit 12 which controls its gain in accordance with a gain control signal so as to keep the average level constant. An output signal of the automatic gain control circuit 12 is converted into a digital signal by an a/d converter 13. The digital signal from the a/d converter 13 is supplied to an average detecting circuit 14 which detects an average level of the digital signal. The average value is supplied to a gain controlling signal generation circuit 15 which generates the gain control signal such that the gain is controlled to keep the average level constant.
On the other hand, an electronic shutter signal generation circuit 16 generates an electronic shutter signal of which shutter speed is fixed to {fraction (1/100)} sec at the 50-Hz-ac area and fixed to {fraction (1/30)} sec or {fraction (1/60)} sec at the 60-Hz-ac area. FIGS. 5A to 5D show prior art video signal processing operation at the 50-Hz-ac area. The frame interval is {fraction (1/30)} sec. On the other hand, the shutter speed is fixed to {fraction (1/100)} sec at that area so that the exposure interval ({fraction (1/100)} sec) agrees with the luminance variation period of fluorescent lamps. Accordingly, as shown in FIG. 5B, the luminance level of the video signal from the MOS type of imager is constant, wherein flicker is suppressed. In this prior art video signal processing circuit, the gain controlling is provided by only the automatic gain control circuit 12, so that the automatic exposure controlling is limited by the dynamic range of the automatic gain controlling circuit 12.